The present invention relates to a video disc type recording and reproducing system for multiplexed video and audio signals, and, more particularly, to such a system in which the influence of sidebands of the frequency-modulated video signal on the frequency-modulated audio signal is eliminated.
Systems in which video information and audio information are recorded on a video disc and are simultaneously reproduced have been known. One thereof is disclosed in U.S. Pat. No. 3,969,756 and is shown in FIG. 1, schematically. In FIG. 1, a video carrier of, for example, 8.2 MHz is frequency-modulated with the video information by a frequency modulator 1 so that the sync level, the pedestal level and the white peak level thereof become, for example, 7.6 MHz, 8.1 MHz and 9.3 MHz, respectively.
At least one audio signal is also frequency-modulated by a frequency modulator 2, and outputs of the frequency modulators 1 and 2 are added by an adder circuit 3. An output of the adder circuit 3 is supplied to a limiter 4 by which an amplitude of the adder output is limited, and pulse width modulation is performed. An output of the limiter is supplied to an electrical/optical (E/O) modulator 5 in which a laser beam from a laser source 6 is modulated by the limiter output, and the resultant signal is recorded on a video disc 7 in a known manner. Upon reproduction, the recorded information is picked up by a pick-up 8 and amplified by an amplifier 9. An output of the amplifier 9 is suitably divided and the resultant video and audio information are detected by detectors 10 and 11, respectively.
FIG. 2a shows an example of the frequency spectrum of the output of the limiter 4. In FIG. 2a, letter A represents the frequency band of the frequency-modulated video signal, B.sub.1 and B.sub.2 are the primary sidebands thereof and C.sub.1 and C.sub.2 are the secondary sidebands thereof. D and E are audio signal carriers when the audio system is a 2-channel or bilingual system. The frequency band width necessary to demodulate the video signal is from 3.9 to 13.5 MHz in the NTSC system in which the primary sidebands are included, because the modulation index is unity or smaller. Therefore, it is usual to set the audio subcarriers at, for example, 2.3 MHz and 2.8 MHz, so that these frequencies are out of the above frequency band necessary for video signal demodulation.
When the video input is composed of a ramp signal combined with a Y (brightness) component, the secondary sideband of the Y component is from 2.6 to 3.5 MHz, which covers at least a portion of the audio subcarriers.
Although the level of the secondary sideband of the video RF signal is lower than that of the primary sideband, it still affects the audio subcarriers, because the level of the audio subcarriers is usually set at around one tenth the video carrier level, and may be further lowered by 6 dB by passage through the limiter 4. Therefore the carrier to noise ratio (C/N) of the frequency-modulated audio signal is degraded, resulting in a degraded S/N ratio and reproduction quality.
Further, in a four channel audio system, a subcarrier of, for example, 47.2 KHz is frequency-modulated with audio signals from 2 additional channels, respectively, and these frequency-modulated subcarriers are added to the main channel signals. Then, the main carriers of 2.3 MHz and 2.8 MHz are frequency-modulated with the resultant signals, respectively. In this case, the effect of the secondary and/or higher sidebands of the, frequency-modulated video signal on the audio signals becomes more severe.
Further, since the dynamic range of the audio information in the conventional video disc type recording and reproducing system, in which an analog audio signal is directly frequency-modulated, is as small as 60-65 dB, it is relatively difficult to obtain a high quality reproduction signal.